Theory and Application of Nonlinear Electrochemical Impedance Spectroscopy (NLEIS)
Traditional electrochemical impedance spectroscopy (EIS) involves measuring the response of an electrochemical system or device to a small-amplitude perturbation. However, a significant limitation of EIS is that it only probes the linearized response, filtering out potentially useful information contained in the nonlinear behavior. Over the last 7 years, our group has pioneered an extension of EIS called nonlinear EIS, (NLEIS) that attempts to capture this lost nonlinear information via measurement of higher-order voltage harmonics. We find that the harmonic signals often contain mechanism-specific features that can help distinguish physical processes or rate laws governing an electrode reaction.
Unlike a Tafel-slope measurement, NLEIS can be applied within a very small polarization window, mitigating voltage-induced hysteretic behavior. NLEIS is also very rubust to drift or degradation of performance because it measures and tracks the instantaneous nonlinear response, as opposed to reconstructing nonlinear behavior from multiple EIS experiments over time (where nonlinearity and time-dependence are convoluted). This talk will review the theory behind NLEIS, and provide practical details about how we implement the measurement, as well as predict responses based on physical models. Examples will highlight results from ongoing studies of porous and thin-film mixed conducting electrodes, including analysis of degradation phenomena and analysis of O2 reduction on inhomogeneous surfaces.